Production of polyenoic fatty acids (fatty acids containing 2 or more unsaturated carbon-carbon bonds) in eukaryotic microorganisms is generally known to require the presence of molecular oxygen (i.e., aerobic conditions). This is because it is believed that the cis double bond formed in the fatty acids of all non-parasitic eukaryotic microorganisms involves a direct oxygen-dependent desaturation reaction (oxidative microbial desaturase systems). Other eukaryotic microbial lipids that are known to require molecular oxygen include fungal and plant sterols, oxycarotenoids (i.e., xanthophyls), ubiquinones, and compounds made from any of these lipids (i.e., secondary metabolites).
Eukaryotic microbes (such as algae; fungi, including yeast; and protists) have been demonstrated to be good producers of polyenoic fatty acids in fermentors. However, very high density cultivation (greater than about 100 g/L microbial biomass, especially at commercial scale) can lead to decreased polyenoic fatty acid contents and hence decreased polyenoic fatty acid productivity. This may be due in part to several factors including the difficulty of maintaining high dissolved oxygen levels due to the high oxygen demand developed by the high concentration of microbes in the fermentation broth. Methods to maintain higher dissolved oxygen level include increasing the aeration rate and/or using pure oxygen instead of air for aeration and/or increasing the agitation rate in the fermentor. These solutions generally increase the cost of lipid production and can cause additional problems. For example, increased aeration can easily lead to severe foaming problems in the fermentor at high cell densities and increased mixing can lead to microbial cell breakage due to increased shear forces in the fermentation broth (this causes the lipids to be released in the fermentation broth where they can become oxidized and/or degraded by enzymes). Microbial cell breakage is an increased problem in cells that have undergone nitrogen limitation or depletion to induce lipid formation, resulting in weaker cell walls.
As a result, when lipid producing eukaryotic microbes are grown at very high cell concentrations, their lipids generally contain only very small amounts of polyenoic fatty acids. For example, the yeast Lipomyces starkeyi has been grown to a density of 153 g/L with resulting lipid concentration of 83 g/L in 140 hours using alcohol as a carbon source. Yet the polyenoic fatty acid content of the yeast at concentration greater than 100 g/L averaged only 4.2% of total fatty acids (dropping from a high of 11.5% of total fatty acid at a cell density of 20-30 g/L). Yamauchi et al., J. Ferment. Technol., 1983, 61, 275-280. This results in a polyenoic fatty acid concentration of only about 3.5 g/L and a polyenoic fatty acid productivity of only about 0.025 g/L/hr. Additionally, the only polyenoic fatty acid reported in the yeast lipids was C18:2.
Another yeast, Rhodotorula glutinus, has been demonstrated to have a lipid productivity of about 0.49 g/L/hr, but also a low overall polyenoic fatty acid content in its lipids (15.8% of total fatty acids, 14.7% C18:2 and 1.2% C18:3) resulting in a polyenoic fatty acid productivity in fed-batch culture of only about 0.047 g/L/hr and 0.077 g/L/hr in continuous culture.
Present inventors have previously demonstrated that certain marine microalgae in the order Thraustochytriales can be excellent producers of polyenoic fatty acids in fermentors, especially when grown at low salinity levels and especially at very low chloride levels. Others have described Thraustochyrids which exhibit a polyenoic fatty acid (DHA, C22:6n-3; and DPA, C22:5n-6) productivity of about 0.158 g/L/hr, when grown to cell density of 59 g/L/hr in 120 hours. However, this productivity was only achieved at a salinity of about 50% seawater, a concentration that would cause serious corrosion in conventional stainless steel fermentors.
Costs of producing microbial lipids containing polyenoic fatty acids, and especially the highly unsaturated fatty acids, such as C18:4n-3, C20:4n-6, C20: 5n3, C22: 5n-3, C22:5n-6 and C22:6n-3, have remained high in part due to the limited densities to which the high polyenoic fatty acid containing eukaryotic microbes have been grown and the limited oxygen availability both at these high cell concentrations and the higher temperatures needed to achieve high productivity.
Therefore, there is a need for a process for growing microorganisms at high concentration which still facilitates increased production of lipids containing polyenoic fatty acids.